This invention relates to detection of leaks in sealed articles and, more particularly, to a novel helium mass spectrometer leak detector.
Helium mass spectrometer leak detection is a known leak detection technique. Helium is used as a tracer gas which passes through the smallest of leaks in a sealed test piece. The helium is then drawn into a leak detection instrument and measured. The most important component of the instrument is a mass spectrometer tube which detects and measures the helium. The input gas is ionized and mass analyzed by the spectrometer tube in order to separate the helium component which is then measured. In one approach, a test piece is pressurized with helium. A sniffer probe connected to the test port of a leak detector is moved around the exterior of the test piece. In the vicinity of a leak, helium is drawn into the probe and is measured by the leak detector. In an alternative approach, the interior of a test piece is coupled to the test port of the leak detector. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak, and is measured by the leak detector. Helium mass spectrometer leak detection provides good sensitivity, a wide dynamic range and is fast and convenient to use.
One of the requirements of the spectrometer tube is that the inlet, through which the helium and other gases are received, be maintained at a relatively low pressure, typically, 2.times.10.sup.-4 Torr. In a so-called conventional leak detector, a roughing pump, a diffusion pump and associated forepump and a cold trap are utilized in a conventional vacuum pumping arrangement to maintain the input of the spectrometer tube at the required pressure. The conventional leak detector provides satisfactory performance under a variety of conditions. The cold trap removes water vapor and other contaminants and permits the operating pressure of the spectrometer tube to be reached. However, this configuration has certain drawbacks. The test port which is connected to the test unit or the sniffer probe must be maintained at a relatively low pressure. Thus, the vacuum pumping cycle is relatively long. Furthermore, in the testing of leaky or large volume parts, it may be difficult or impossible to achieve the required pressure level. If the required pressure level can be achieved at all, the pumping cycle is lengthy. As a result, production line testing can be costly. Backstreaming of oil from mechanical pumps into the test piece has also been a problem in conventional leak detectors.
Many of these problems were eliminated in the counterflow leak detector disclosed by Briggs in U.S. Pat. No. 3,690,151. In this leak detector, the mass spectrometer tube is connected to the inlet of a diffusion pump; and the helium tracer gas is introduced through the foreline, or normal output port, of the diffusion pump. The diffusion pump exhibits a high pressure ratio for heavier gases but a low pressure ratio for lighter gases such as helium. Therefore, helium diffuses at an acceptable rate in a reverse direction through the diffusion pump to the spectrometer tube and is measured. Heavier gases in the sample are, to a large degree, blocked by the diffusion pump and prevented from reaching the spectrometer tube. Due to the method of reverse flow in the diffusion pump, the leak detector test port can be operated at the pressure required at the diffusion pump foreline. This pressure is several orders of magnitude higher than the required operating pressure of the spectrometer tube. Performing leak tests at a higher test pressure is advantageous for several reasons. Leaky or large volume pieces can be tested, since the test pressure is relatively easy to attain. Furthermore, vacuum pumping of the test piece, regardless of its size or leak rate, is faster. It was generally thought that, due to the higher operating pressure of the counterflow leak detector as compared with the conventional leak detector, a cold trap would not be beneficial in such a system (see, for example, U.S. Pat. No. 3,690,151). The counterflow-type leak detector has provided highly satisfactory operation under a wide variety of conditions. However, it has certain limited drawbacks. In some cases, backstreaming of oil from the holding or foreline pump into the test piece has occurred. Furthermore, when it is desired to rapidly test certain large, leaky or contaminated parts, or when the humidity level has been high, the capabilities of the roughing pump to reach the required pressure level have been strained and the pumping time has been long. It is desirable to alleviate these problems without affecting the highly desirable qualities of the counterflow leak detector.
It is a general object of the present invention to provide a novel helium mass spectrometer leak detector.
It is another object of the present invention to provide a leak detection system having the features of high test pressure and high water vapor pumping capability.
It is yet another object of the present invention to provide a leak detection system wherein backstreaming of oil into the test piece is eliminated.
It is still another object of the present invention to provide a leak detection system with improved operating speed.